Metal halide lamp containing mercury, light emitting metal, sodium and another alkali metal

ABSTRACT

In a metal halide arc discharge lamp having an arc tube fill including mercury, halogen, sodium and a light emitting metal, controlled amounts of cesium, potassium or rubidium are added to the arc tube fill.

United States Patent Bamberg et a1.

[4 1 Sept. 25. 1973 GTE Sylvania Incorporated, Danvers, Mass.

Filed: Jan. 27, 1972 Appl. No.: 221,222

Assignee:

U.S. Cl. 313/229 H01j 61/18 Field of Search 313/229, 184

[56] References Cited UNITED STATES PATENTS 3,398,312 8/1968 Edris etall 313/229 X 3,514,659 5/1970 Gungle et a1. 3,445,719 5/1969 Thouret eta1 313/229 X Primary ExaminerPalmer C. Demeo Att0meyNorman J.Malley eta1.

[57] ABSTRACT In a metal halide arc discharge lamp having an arc tubefill including mercury, halogen, sodium and a light emitting metal,controlled amounts of cesium, potassium or rubidium are added to the arctube fill.

2 Claims, 1 Drawing Figure METAL HALIDE LAMP CONTAINING MERCURY, LIGHTEMITTING METAL, SODIUM AND ANOTHER ALKALI METAL BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to metalhalide arc discharge lamps. Such lamps comprise an arc tube, usuallymade of fused quartz or other high silica glass, having electrodesdisposed therewithin and containing a fill including mercury and a metalhalide.

2. Description of the Prior Art Metal halide arc discharge lamps of thetype relating to this invention are shown in the following U. S. Pat.Nos.: 3,234,421; 3,250,940; 3,262,012; 3,279,877;

3,569,766; 3,575,630: and 3,634,721.

Such lamps usually contain a fill of mercury, metal halide and a smallamount of ionizable gas. As pointed out in the above patents, metalhalide lamps are more efficient and can yield a whiter light thanmercury vapor lamps. Disclosed in the above patents as metal halidesthat can be added to are tubes of metal halide lamps are the following:lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium,calcium, strontium, barium, scandium, yttrium, lanthanum, titanium,zirconium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum,rhenium, iron, ruthenium, cobalt, rhodium, iridium, nickel, palladium,zinc, cadmium, gallium, indium, thallium, germanium, tin, lead,antimony, bismuth, selenium, tellurium, cerium, praseodymium, neodymium,samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium,ytter bium, lutetium, thorium, and uranium.

In metal halide lamps of general commercial use, an alkali halide,mercury and one or more of the above mentioned metal halides arenormally added. An alkali halide is added to obtain proper arc controland sodium is the preferred alkali since it yields higher efficiency andmore chromatically satisfactory light than either potassium, rubidium orcesium.

In U. S. Pat. No. 3,234,421 the use of halides of three alkali metalslithium, sodium and cesium is disclosed. For maximum efficiency, therecommenced amount of said metals is that amount that will yield apartial pressure during normal lamp operation of l to 200 mm of mercurypressure.

In U. S. Pat. No. 3,262,012 the use of sodium or cesium in an amount of0.53 X 10 to 6.8 X 10" gram atoms per centimeter of arc length isdisclosed for the purpose of establishing stable are conditions in metalhalide lamps.

ln U. S. Pat. No. 3,331,982 the addition of 5.25 X to 6.8 X 10' gramatoms of sodium, lithium, potassium, rubidium and/or cesium percentimeter of arc length is disclosed for the purposes of colorcorrection, arc stabilization and voltage drop reduction.

In U. S. Pat. No. 3,514,659 the use of cesium iodide is disclosed forthe purpose of reducing reignition voltage. The patent points out thatcesium iodide reduces reignition voltage to a greater and more uniformextent than either potassium, lithium or sodium iodide.

In U. S. Pat. No. 3,521,110, alkali metal halides are disclosed as lightemitting metal constituents of metal halide lamps that yield efficient,chromatically satisfactory light, although it is disclosed therein thatthe halides of sodium, potassium and rubidium can result in wallblackening.

There is no disclosure or suggestion in the patents listed above thatparticular alkali metal combinations can result in longer life metalhalide lamps.

SUMMARY OF THE INVENTION As disclosed in the prior art, sodium halide ispreferred over other alkali metal halides as a constituent of the arctube fill of a metal halide lamp because of its improved efficiency andchromaticity. However, we have discovered that the addition of cesium,potassium and/or rubidium halide to a sodium halide containing lampsubstantially increases the life of the lamp under normal operatingconditions. Moreover the amount of added cesium, potassium and/orrubidium halide must be between about 2 and 20 mole percent of theamount of sodium halide in order to obtain longer lamp life withoutsignificantly reducing the efficiency of the lamp or impairing thechromaticity of the light emitted therefrom.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is a perspective viewof a metal halide lamp containing a fill of materials in accordance withthis invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in the drawing, a metalhalide lamp in accordance with this invention includes a generallytubular outer bulbous envelope 1 having a bulbous central portion and aconventional base 14 attached to the bottom thereof. Extending inwardlyfrom the base and inside of the envelope 1 is a mount 15 having a pairof stiff lead-in wires 12 and 16 in electrical conducting relation withthe base 14. Disposed upon one of the stiff lead-in wires 12 is a lower,U-shaped support 8 welded thereto. U-shaped support 8 comprises a pairof vertical wires 23 and 24 rising from a horizontal base wire 25. Theupper ends of lower U-shaped support 8 are welded together with a lowerstrap 7 which in turn supports an arc tube 2. Preferably, the lowerstrap includes two sections abutting against either side of arc tube 2thereby holding it firmly in place. They touch only the press seal ofthe arc tube and not the body. Generally, both sides of the lower strap7 can be of identical construction. A pair of bumpers 26 are welded tolower U- shaped support 8 and abut against the tubular portion of wallsof outer bulbous envelope 1 thereby stabilizing the structure within thelamp. Preferably, these bumpers are made of a resilient material so thatif the lamp is jarred they will absorb much of the shock.

Since lower U-shaped support 8 is electrically connected to stifflead-in wire 12, support 8 forms part of the circuit in the device.Current passes from base 14 into lower U-shaped support 8 and thence tolead-in wire 21 which in turn is connected to a cathode 4 in the arctube. It is sometimes desirable to place an insulating shield aboutlead-in wire 21 to prevent arcing within the lamp and between thevarious elements. Current passes from lead-in wire 21 to cathode 4through an intermediary molybdenum foil section 6.

The other side of the circuit is formed through stiff lead-in wire 16which is preferably bent out of place so that parts on one side of theline are insulated from those on the other side. A resistor 13 isattached to stiff lead-in wire 16 through a lead-in wire associatedtherewith and thence to a connector 27 which in turn leads through amolybdenum foil section 6 to a starting probe 5. A bimetal 22 isdisposed between lead-in wire 21 attached to cathode 4 and lead-in wire27 which is at tached to starting probe 5. Bimetal 22 is biased openwhen the lamp is turned off but when the lamp starts, it biases closedagainst the lead-in wires to probe 5 thereby establishing the samecurrent potential at probe 5 and cathode 4. Such closing preventselectrolysis between the probe and cathode.

At the other end of arc tube 2, an upper support is mounted within thetubular portion of bulbous envelope I. Support frame 10 includes ahorizontal section 18 having vertical supports 17 and 19 dependingdownwardly therefrom and attached at the free ends to an upper strap 11which surrounds the press seal of arc tube 2 and rigidly holds it inplace. Preferably, the construction and disposition of upper strap 11 issimilar to lower strap 7. A pair of upper bumpers 9 are mounted uponvertical sections 17 and 19 of upper support 10 and resiliently abutagainst the sides of the tubular portion of bulbous envelope 1. Suchdisposition prevents breakage of the lamp if the arc tube is shaken ordropped.

A lead-in wire 28 extends to the outside of arc tube 2 and is attachedat its inner end to a molybdenum foil section 6 and thence to a cathode3. An electrical connection is made between stiff lead-in wire 16 andleadin wire 28 through a thin conducting lead 20 which may be of anysuitable conducting material. Preferably, conducting lead 20 isdistantly removed from are tube 2, generally by bending it around theperimeter of outer bulbous envelope 1.

Disposed within arc tube 2 is the usual inert starting gas, such asneon, argon, xenon and the like, and a filler substance 30 whichgenerally exists in the form of a globule during quiescent non-operatingcondition of the lamp. Filler 30 is composed primarily of mercury andincludes, in addition, a light emitting metal in element or compoundform, sodium halide and a halide of cesium, potassium or rubidium.Mercury is present in the usual amount, that is to say, so that undernormal operating conditions, the mercury is totally volatilized andproduces a pressure within arc tube 2 of about k to 10 atmospheres. Thelight emitting metal can be one or more of scandium, indium, thallium,gallium, dysprosium or any light emitting metal commonly used in metalhalide lamps, such as disclosed in the patents cited above underDescription of the Prior Art.

Sodium halide is present in an amount that will stabilize the arc and/oryield chromatically pleasing light, as mentioned previously. Such amountis generally about 0.] to 3.0 milligrams per cubic centimeter of arctube volume or about 5 X 10" to 7 X 10 gram atoms per centimeter of arclength, although, as pointed out in Waymouth, J. F., Electric DischargeLamps, Mass., M.l.T. Press, 1971, pages 23 l 266, most of the sodiumhalide is present in a condensed phase during normal lamp operation.

A halide of cesium, potassium or rubidium is present in an amount thatis between about 2 and 20 mole percent of the quantity of sodium halide.There is no significant increase in lamp life when said amount is lessthan about 2 mole percent; and above about 20 mole percent, lampefficiency is reduced to a commercially unacceptable level.

In U. S. Pat. No. 3,424,935 the problem of sodium migration through anarc tube wall is disclosed. It is believed that increased lamp liferesults from the instant invention because cesium, potassium andrubidium migrate through the arc tube wall at a considerably lower ratethan does sodium. Thus after, say, several thousand hours of lampoperation, where there might be insufficient sodium halide remaining inthe arc tube fill to prevent arc restriction, there is sufficientcesium, potassium or rubidium halide present to prevent are restriction.Arc restriction can cause premature lamp failure by local overheating ofthe arc tube.

The migration rates of sodium, cesium, potassium and rubidium in 400watt lamps were compared under accelerated life test conditions. Charge30 in all the lamps consisted of 50 mg mercury, 5 mg mercuric iodide,0.5 mg scandium and, in respective order 20 mg sodium iodide, 18 mgsodium iodide plus 2 mg cesium iodide, 18 mg sodium iodide plus 2.1 mgpotassium iodide, and 18 mg sodium iodide plus 3 mg rubidium iodide. Themigration rates of cesium, potassium and rubidium were found to benegligible while the sodium migration rate was substantial.

In another accelerated life test on 400 watt metal halide lamps, charge30 contained 50 mg of mercury, 5 mg of mercuric iodide, 0.5 mg ofscandium and 1.3 X 10 gram atoms (equal to 3 X 10' gram atoms per cm ofarc length) of alkali iodide. In one set of lamps the alkali iodide wassodium iodide; in a second set of lamps it was 95 percent mole percentsodium iodide and 5 mole percent cesium iodide; in the third and fourthsets of lamps the cesium iodide concentration was 15 and 25 mole percentrespectively. The lamps containing 25 mole percent cesium iodide had anefficiency less than lumens per watt while the lamps containing 0percent, 5 percent and 15 percent cesium iodide all had efficienciesbetween and lumens per watt. After hours of accelerated life testing,the lamps without cesium iodide failed, while all lamps with cesiumiodide continued to operate satisfactorily.

We claim:

1. A metal halide arc discharge lamp comprising: an arc tube havingsealed ends and electrodes disposed in said ends; and a fill within saidare tube comprising mercury, a light-emitting metal, sodium halide inthe amount of about 5 X 10" to 7 X 10 gram atoms per centimeter of arclength and a quantity of an arc-restriction-preventing metal halidewhich diffuses through the arc tube wall at a significantly lower ratethan sodium and thus prevents arc restriction in an operating lamp aftersodium, because of gradual diffusion through the arc tube wall, nolonger effectively prevents arc restriction, saidarc-restriction-preventing metal halide being cesium, potassium orrubidium, said quantity being between 2 to 20 percent of sodium halide.

2. The lamp of claim 1 wherein said light emitting metal is scandium,indium, thallium, gallium or dysproslum.

l I t

2. The lamp of claim 1 wherein said light emitting metal is scandium,indium, thallium, gallium or dysprosium.